The present invention relates to an improvement in an apparatus for controlling an elevator driven by an AC electric motor.
There is known an elevator control apparatus, in which an induction motor is employed for driving an elevator cage and wherein a variable-voltage and variable-frequency AC power converted by an inverter is supplied to the motor to control the speed thereof. FIG. 1 is a block diagram of such an elevator control apparatus.
In FIG. 1, reference numeral 1 denotes a power switch for turning on/off a three-phase AC source R, S and T; 2 denotes a converter connected to the power switch 1 and constituted by thyristors for rectifying the source voltage into a DC voltage; 3 denotes a smoothing capacitor connected to the DC side of the converter 2; 4 denotes a known inverter connected to the smoothing capacitor 3 and constituted by transistors and diodes for converting the DC power into AC power having a desired voltage and frequency. Reference numeral 5 denotes a control circuit for controlling the converter 2 and the inverter 4; 6 denotes a three-phase induction motor connected to the AC side of the inverter 4 for hoisting the cage; 7 denotes a hoist driving sheave driven by the motor 6; 8 denotes a main rope wound around the sheave 7; and 9 and 10 denote an elevator cage and a balance weight, respectively, connected to opposite ends of the main rope 8.
The three-phase AC power is thus converted by the converter 2 and the smoothing capacitor 3 into DC power which is in turn supplied to the inverter 4. The inverter 4 converts the supplied DC power into variable-voltage and variable frequency AC power which is in turn supplied to the motor 6. This operation is controlled by the control circuit 5. Thus, the motor 6 is driven and the cage is caused to move upward/downward.
However, since a surge current flows into the capacitor 3 upon the turn-on of the power switch 1, it is necessary to control the current to limit the same to a value below the rated current value of the smoothing capacitor 3 and the converter 2. For this purpose, the control circuit 5 should be arranged so as to control the firing angle of the thyristors in the converter 2.
For the smoothing capacitor 3, an electrolytic capacitor having a large capacitance is usually employed. Generally, the lifetime of such an electrolytic capacitor is shorter than that of the elevator, which is about 20-30 years, while this of course depends on the use conditions. In this case, therefore, the reduction in capacitance of the capacitor affects elevator control.